There has been a strong demand for reduction of emissions of carbon dioxide in order to deal with global warming. In the automobile industry, the reduction of emissions of carbon dioxide is highly expected in association with the spread of electric vehicles (EV) and hybrid electric vehicles (HEV). Thus, development of secondary batteries for driving motors as a key to practical application of such vehicles, is actively being carried out.
Lithium-ion secondary batteries for driving motors are required to have quite high output characteristics and high energy in comparison with generally-used lithium-ion secondary batteries for use in, for example, mobile phones and laptop computers. Thus, lithium ion secondary batteries having relatively high theoretical energy are gaining increased attention among other types of batteries and development thereof is being made rapidly.
A lithium ion secondary battery generally has a configuration in which a positive electrode and a negative electrode are connected via a separator containing an electrolyte, which are housed in a battery case. The positive electrode is obtained in a manner such that a positive electrode active material is applied to both surfaces of a positive electrode current collector by use of a binder. The negative electrode is obtained in a manner such that a negative electrode active material is applied to both surfaces of a negative electrode current collector by use of a binder. The separator is required to not only hold an electrolysis solution to ensure lithium ion conductivity between the positive electrode and the negative electrode but also serve as a partition between the positive electrode and the negative electrode. In addition, the separator preferably has a function to prevent movement of lithium ions (a shutdown function) to stop a charge-discharge reaction when the battery reaches a high temperature. As for such a separator, a microporous film containing thermoplastic resin such as polyolefin is conventionally used.
When the separator containing a soft material such as polyolefin is used, however, foreign body pieces entering at the time of manufacturing the battery or electrode active material layer pieces exfoliated from the electrodes, penetrate the separator, which may cause an internal short circuit.
In view of such a problem, for example, Japanese Patent Unexamined Publication No. 11-080395 teaches that a surface protection layer containing inorganic fine particles and having ventilation is formed at least on one surface of a film having a porous structure so as to prevent an internal short circuit.